The evolving threats posed by concealed explosives or the intentional release of toxic chemicals demand new ways to detect these threats and protect the public. Typically, the techniques for identifying threat molecules involve ionizing a sample and then detecting whether the threat molecule (analyte) is present. The detection mechanisms include ion mobility spectrometry (IMS), differential mobility spectrometry (DMS), field asymmetric ion mobility spectrometry (FAIMS), and mass spectrometry (MS), all of which rely upon ionization of the analyte or a complex that includes the analyte. In fact, one of these techniques (IMS) is currently used in nearly every airport in the United States as a means to prevent concealed explosives from getting on aircraft.
Given the importance of these techniques to public safety, considerable effort has been devoted to develop better techniques for efficiently (and selectively) ionizing analytes in order to provide the greatest detection capability.
In almost all instances, ionization is achieved selectively by performing the ionization under ambient-pressure conditions in the presence of an ionization reagent in a technique known as ambient-pressure ionization (API) (also sometimes called atmospheric-pressure chemical ionization). In API, the target analyte is drawn into a space containing both an ionization source and an ionization reagent, and ionization of the target molecule takes place through ion-molecule collisions. The ionization reagent is selected such that rapid achievement of charge equilibrium results in charge or proton transfer from the reagent to the target molecule.
Since many explosive and chemical threats have low vapor pressure and exist as traces of particulates or thin films on surfaces, the most common way to collect a sample requires a swipe or swab substrate which provides a physical mechanism to both collect and preconcentrate a sample taken from a surface of a suspect object for subsequent presentation to the ionization space of the detection instrument. The substrate media, which is called a “swipe,” can be thermally heated to desorb the target analyte into the vapor phase for subsequent ionization and detection. This methodology is currently used in fielded IMS systems that detect explosives, where detection relies on efficient collection and presentation of low-vapor analytes such as 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and pentaerythritol tetranitrate (PETN) into the instrument, and use of ionization reagents that enhance the formation of negative ions via chloride adduction, such as methylene chloride.
In such explosive detection systems, the swipe or substrate is typically positioned in a thermal desorber located on the inlet side of the detection system. Thermal heating of the solid particles on the swipe induces a solid-to-vapor phase transition and releases the analyte molecules as a vapor, usually guided into the sensor inlet by a carrier gas, and the ionization reagent is introduced as a vapor within a separate carrier gas. Properties of commercially-available swipe media have been optimized over the years for increased efficiency of particle collection from surfaces (mechanical or electrostatic), efficient transfer and release of analyte into the chemical sensor, thermal stability, and low chemical background of the substrate.
Detection of both inorganic and organic oxidizer-based explosives can be a particularly difficult problem. Some examples of inorganic oxidizer-based explosives include perchlorate (KClO4, NaClO4), chlorate (KClO3, NaClO3), and nitrate (NH4NO3, KNO3, NaNO3) salts and hydrogen peroxide (H2O2). Examples of organic oxidizer-based explosives are hexamethylene triperoxide diamine (HMTD), triacetonetriperoxide (TATP), and diacetonediperoxide (DADP). Inorganic oxidizers, generally speaking, are chemical compositions that contribute oxygen in which the fuel component of an explosive can burn. Two factors that contribute to the difficulty in detecting inorganic oxidizers are their low vapor pressure and low ionization yield. Low volatility analytes require high thermal desorption and/or ionization source temperatures. (In some cases the temperature necessary to transform oxidizer analytes into their vapor phase can exceed 350° C.—a regime in which common swipe materials cannot be used.) Achieving high temperature is an engineering challenge specifically in smaller, field portable systems where size, weight and power must be minimized and long thermal cycling reduces sample throughput. Even when ionized, this class of analytes are often prone to quickly recombine into neutral species before they can be subjected to spectrometric analysis. Moreover, some of the analytes also form ubiquitous, non-specific products upon thermal desorption, e.g. nitrate from ammonium nitrate, potassium nitrate, and sodium nitrate.
Accordingly, there exists a need for better methods and reagents for detecting oxidizer compositions and oxidizer-based explosives. Reagents that can improve desorption (release of analytes from a substrate), increase the quantity or longevity of ionized analyte species or otherwise improve the detector efficiency would satisfy a long-feel need in the field. Additionally, indirect techniques for detecting or quantifying oxidizer analytes based on formation of complexes or chemical modification of reagents, which can be more readily detected, would also provide an improvement in the art.